Electrogravimetry and coulometry Dr Bhagure G R Department

- - - Electrogravimetry and coulometry Dr. Bhagure G. R. Department of Chemistry

Introduction, Electrogrvimetry Factors affecting the nature of the deposit, Instrumentation Applications

• Electrogrvimetry – The product is weighed as a deposit on one of the electrodes (the working electrode) • Involve deposition of the desired metallic element upon a previously weighed cathode, followed by subsequent reweighing of the electrode plus deposit to obtain by difference the quantity of the metal • Cd, Cu, Ni, Ag, Sn, Zn can be determined in this manner • Few substances may be oxidized at a Pt anode to form an insoluble and adherent precipitate suitable for gravimetric measurement e. g. oxidation of lead(II) to lead dioxide in HNO 3 acid

• Some Basic concepts

CONDUCTO R metallic conductor electrolytic conductor

Electrochemical Cells : Galvanic: Chemical energy into electrical energy : Spontaneous redox reaction Electrolytic Cells: Electrical energy into Chemical energy non- Spontaneous redox reaction Cells

The anode of an electrolytic cell is positive Electrolytic Cells cathode is negative), since the anode attracts anions from the solution

Electrochemical Cells Potentiometric measurements are made in the absence of current flow. The measured potential is that of a galvanic or voltaic cell.

Electrolytic Cells Chemical analyses can also be based on using a cell in an electrolytic fashion (i. e. , driving a reaction with an applied voltage). In these cases, current flows in the cell.

E APPLIED = E CELL NO CURRENT FLOWS IN CELL Electrolysis does not takes place Possibility for electrolysis E APPLIED > E CELL CURRENT FLOWS IN CELL ELECTROLYSIS TAKES PLACE

cell potential To drive an endothermic electrochemical reaction, the applied voltage, Applied, must overcome the IR drop

IR drop • Note that an electrochemical cell has a resistance to current flow, in analogy with resistance to current flow in any conductor. The relationship between voltage, current, and resistance is ohm’s Law: • E=I R , where E is potential (voltage), I is current (in amperes) and R is resistance (in ohms)

A voltaic cell in a circuit consists of : 1) a negative electrode -anode 2) a positive electrode - cathode 3) an electrolyte Anode Zn e – V + Cathode Cu Zn. SO 4 Porous fritted disk (liquid junction) Cu. SO 4

Electrolytic Cells

current density The factors influencing the physical characteristics of deposits are temperature The effects of temperature are unpredictable and must be determined empirically presence of Complexing agents Many metals form smoother and more adherent films when deposited from solutions in which their ions exist primarily as complexes e. g. Cyanide and ammonia complexes

• Principles of electrolysis • Electrolysis is the process in which a reaction is driven in its no-spontaneous direction by the application of an electric current.

Nernst Potential Std. Oxidation Potential Std. Reduction Potential

OHAMIC (SOLUTION) POTENTIAL Non-nernstian potentials : CONCENTRATION POLARIZATION OVER POTENTIAL/ Voltage

Principles of electrolysis Electrolysis is the process in which a reaction is driven in its nonspontaneous direction by the application of an electric current. Endergonic reaction G>0 NOTE: electrolysis is the process of driving an electrochemical reaction in its non‑spontaneous direction through the application of voltage/current. To accurately assess voltage-current relationships, we must consider some sources of non-Nernstian potentials : Ohmic (solution) potential, Concentration polarization, Overpotential.

Electrolysis experiment § Direct current (dc) is current that is always in one direction; it is unidirectional. § The direction of alternating current (ac) reverses periodically. DC voltage sources are often given the battery symbol with + and – polarities. An arrow through the battery indicates that the source voltage is variable and can be changed to another dc value. Cathode (working electrode): 2 Cu 2+ + 2 e = Cu(s) Anode (counter electrode): H 2 O = ½O 2(g) + 2 H+ + 2 e Net reaction: H 2 O + Cu 2+ = Cu(s) + ½O 2(g) + 2 H+

An electrolytic cell for determining Cd 2+. (a) Current=0. 00 m. A. (b) Schematic of cell in (a) with internal resistance of cell represented by a 15. 0Ω resistor and Eapplied increased to give a current of 2. 00 m. A. Ag | Ag. Cl(s), Cl– (0. 200 M), Cd 2+ (0. 00500 M) | Cd Cd 2+ + 2 Ag(s) + 2 Cl– Cd(s) + 2 Ag. Cl(s) Reduction Working electrode operates as a cathode when apply a potential somewhat more negative than a thermodynamic potential of – 0. 734 V.

Current and potential changes during an electrolysis. Whenever current flows, three factors act to decrease the output voltage of galvanic cell or to increase the applied voltage needed for electrolysis. 1) Ohmic potential ; Ohmic drop The voltage needed to force current (ions) to flow through the cell. Eohmic = IR The output voltage of a galvanic cell is decreased by IR. Egalvanic = Eequilibrium – IR The magnitude of the applied voltage for an electrolysis cell must be more negative than thermodynamic cell potential by IR in order for current flow. Eapplied = Ecathode – Eanode – IR = Ecell – IR

2) Polarization effects I = (Ecell – Eapplied) / R = – (Eapplied / R) + (Ecell / R) Overvoltage ( ) is the potential difference between theoretical cell potential from Eapplied = Ecell – IR and the actual cell potential at a given level of current. Eapplied = Ecell – IR – The term polarization refers to the deviation of the electrode potential from the value predicted by the Nernst equation on the passage of current. Cells that exhibit nonlinear behavior at higher currents exhibit polarization, and the degree of polarization is given by an overvoltage or overpotential.

Experimental current/voltage curve for operation of the cell shown in Figure 22 -1. Dashed line is theoretical curve assuming no polarization. Overvoltage ∏ is the potential difference between theoretical curve and the experimental.

Factors that influenced polarization q Electrode size, shape, and composition q Composition of the electrolyte solution q Temperature And Stirring Rate q. Current Level q Physical state of species involved in the cell reaction Two categories of polarization phenomena 1) Concentration polarization 2) Kinetic polarization

Concentration Polarization Concentration polarization occurs because of the finite rate of mass transfer from the solution and an electrode surface. The electrode potential depends on the concentration of species in the region immediately surrounding the electrode. When ions are not transported to or from an electrode as rapidly as they are consumed or created, we say that concentration polarization exists. That is, concentration polarization means that [X]s [X]o, , where [X]o is the concentration of X in the bulk solution and [X]s is concentration of X in the immediate vicinity of the electrode surface. Reactants are transported to and products away from an electrode by three mechanisms: (1) diffusion, (2) migration, (3) convection (as a result of stirring, vibration, or temperature gradients) To decrease concentration polarization : (1) Raise the temperature. (2) Increase stirring (3) Increase electrode surface area. (4) Change ionic strength

Concentration Polarization Pictorial diagram (a) and concentration vs. distance plot (b) showing concentration change at the surface of a cadmium electrode. As Cd 2+ ions are reduced to Cd atoms at the electrode surface, the concentration of Cd 2+ at the surface becomes smaller than the bulk concentration. Ions then diffuse from the bulk of the solution to the surface as a result of the concentration gradient. The higher the current, the larger the concentration gradient until the surface concentration falls to zero, its lowest possible current, called the limiting current, is obtained.

Current-potential curve for electrolysis showing the linear or ohmic region, the onset of polarization, and the limiting current plateau. In the limiting current region, the electrode is said to be completely polarized, since its potential can be changed widely without affecting the current.

Migration involves the movement of ions through a solution as a result of electrostatic attraction between the ions and the electrodes. Migration of analyte species can be minimized by having a high concentration of an inert electrolyte, called a supporting electrolyte, present in the cell. The motion of ions through a solution because of the electrostatic attraction between the ions and electrodes is called migration.

Electrogravimetric analysis Electrodeposition analysis in which the quantities of metals deposited may be determined by weighing a suitable electrode before and after deposition. (a) Electrogravimetric analysis. Analyte is deposited on the large Pt gauze electrode. If analyte is to be oxidized rather than reduced, the polarity of power supply is reversed so that deposition still occurs on the large electrode. Apparatus for electrodeposition of metals without cathode-potential control. Note that this is a two-electrode cell. (b) Outer Pt gauze electrode. (c) Opened inner Pt Pt gauze electrode designed to be spun by a mortor in place of magnetic stirring.

Tests for completion of the deposition : 1) disappearance of color 2) deposition on freshly exposed electrode surface 3) qualitative test for analyte in solution In practice, there may be other electroactive species that interfere by codeposition with the desired analyte. Two general types of electrolytic procedures : 1) Electrogravimetry without potential control 2) Controlled-potential ; potentiostatic method

Types of Electro gravimetric Analysis Constant Current Electrolysis Constant Potential Electrolysis

The apparatus Consists of a suitable cell for the purpose of electrolysis A 6 V STORAGE BATTERY Direct current source AN AMMETER Used to indicate the current Voltmeter Constant Current Electrolysis Resistor. Used to indicate the applied THE voltage VOLTAGE APPLIED TO THE CELL IS CONTROLLED BY A RESISTOR.

Na+ Cl-

Constant Potential Electrolysis: By controlled potential electrolysis, it is possible to separate two elements whose deposition potentials differ sufficiently (by a few tenths of a volt). The potential of the cathode is controlled so that it never becomes sufficiently negative to allow the deposition of the next element. The potential of the cathode becomes negative (due to concentration polarisation) and that co-deposition of the other species begins before the analyte is completely deposited. A large negative drift in the cathode potential can be avoided by using a three electrode system as shown in Fig.

Constant Potential Electrolysis Working electrode : where the analytical reaction occurs. Auxiliary electrode : the other electrode needed for current flow Reference electrode (SCE) : used to measure the potential of the working electrode

• A potentiostat maintains the working electrode potential at a constant value relative to a reference electrode. • The electrolysis current passes between the working electrode and a counter electrode. The counter electrode has no effect on the reaction at the working electrode. • Reduction reactions occur at working electrode potentials ( measured with respect to the reference electrode ) that are more negative than that required to start the reaction. Oxidations occur when the working electrode is more positive than necessary to start the reaction. • Controlled potential means that a constant potential difference is maintained between the working and reference electrodes. • Constant voltage means that a constant potential difference is maintained between the working and auxiliary electrodes. • Controlled potential affords high selectivity, but the procedure is slower than constant voltage electrolysis.

Charges in cell potential (A) and current (B) during a controlled-potential deposition of copper. The cathode is maintained at – 0. 36 V (vs. Lingane, Anal. Chem. . Acta, 1948, 2, 590. )

A mercury cathode for the electrolytic removal of metal ions from solution.

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